The present invention relates generally to a communication system and, more particularly, to a communication system for communicating information via a power distribution network.
Geometric harmonic modulation (GHM) allocates signaling energy into lobes, tones or different evenly spaced frequencies. The GHM tones have frequencies that are midway between a pair of adjacent power line harmonic frequencies. With the GHM tones midway between the harmonic frequencies, the harmonic center frequencies coincide with first nulls of the GHM tones and vice versa.
Typically, a fast Fourier transform (FFT) is used to channelize a received GHM signal into frequency bands with a 30 hertz (Hz) spacing. This bin spacing is obtained using a 1024 bin FFT with sampling rate of about 30720 Hz. The objective is to separate the GHM signal from the harmonics of 60 Hz that are present in the power line. By using a bin spacing of 30 Hz, these harmonics can theoretically be eliminated by discarding every other bin of the FFT. At the same time, the GHM carriers are selected to be mid-way between the harmonics of 60 Hz so that the harmonics can be removed and the GHM signal will remain. Also, the data rate is chosen to be 30 Hz so that each modulated GHM carrier can be represented by a signal bin of the FFT.
However, each bin of the FFT does not have an ideal bypass frequency response. Rather, an ideal bypass frequency can have a       sin    ⁢          xe2x80x83        ⁢    χ    χ
response with nulls occurring at the other bin centers. Consequently, if the harmonics of the power line signal are exactly at multiples of 60 Hz and the sampling frequency is exactly 30720 Hz, the harmonics will have no effect on the frequency bins in which the GHM signal is located. However, in practice, noise and frequency variations on the power line prevent maintaining an ideally perfect frequency lock.
In one exemplary embodiment of the present invention, a communication system comprises a transmitter that is configured to transmit a GHM signal. The transmitter is configured to pulse shape the GHM signal to confine the frequency spectrum of the GHM signal to within a predefined frequency range. A receiver is provided that receives the transmitted GHM signal. In another representative embodiment, the transmitter includes a root raised cosine filter that pulse shapes the GHM signal transmissions to have the same frequency response as a matched filter in the receiver. In even another representative embodiment of the present invention, the receiver comprises a processor that maximizes the signal to noise ratio and minimizes the bit error rate. The processor is configured to estimate the amplitude distortion resulting from transmission of the GHM signal on via a power line distribution system.
In yet another representative embodiment of the present invention, a method is provided for synchronization of GHM signals to establish relative times for the correct interpretation of bit positions. The method comprises the steps of transmitting signals from a transmitter to a receiver that is coupled to a power line. The signals are synchronized to establish a relative time for correct interpretation of the bit positions. A data frame is generated including a preamble configured with a Barker sequence. The data frame is transmitted from the transmitter to a power line. The receiver receives the transmitted data frame. The presence of the Barker sequence is detected in the transmitted data frame.